The present invention relates to a fuel supply system for an internal combustion engine.
Using conventional technology, a system has been devised for heating fuel, which is injected from an upstream fuel injection valve, by means of a heater located in an air intake passage to vaporize it, thereby reducing fuel accumulating in the air intake passage or an air intake valve, resulting in improving combustion especially at the cool engine start-up period and also reducing displacement of toxic hydrocarbon.
For example, such a system comprising a vaporized-fuel supply system, which heats fuel injected from a fuel injection valve and vaporizes it, has been disclosed in U.S. Pat. No. 5,894,832. The disclosed system is designed such that a downstream fuel injection valve is located near the air intake port of each cylinder, and an upstream fuel injection valve and a heater are located in an auxiliary air passage which bypasses an upstream throttle valve so that fuel from the upstream fuel injection valve is injected toward the heater during the engine warm-up period and is vaporized by the heater, thereby preventing fuel from accumulating in the air intake passage resulting in improved combustion.
However, according to the conventional technology, if fuel is supplied from a fuel vaporizer located upstream of the air intake passage at the engine start-up time, a large volume of air located downstream of the throttle valve in the air intake pipe is supplied to the cylinder prior to the supply of the vaporized fuel. Accordingly, a problem arises in that the time for start-up becoming longer than an MPI system, for example, which does not apply a conventional fuel vaporizer.
Furthermore, when vaporized-fuel begins to be supplied, several cylinders do not receive the necessary predetermined combustible air-fuel ratio; therefore, creating another problem of the occurrence of flameout which prevents the reduction of HC.